Fungsi Anchor

Ini listingnya

 <?php
                    $main_menu = $this->db->get_where('tabel_menu',array('is_main_menu'=>0))->result();
                    foreach ($main_menu as $main) {
                      echo "<li>".anchor($main->link,$main->nama_menu)."</li>";
                    }
                  ?>

Jadi ketika pemanggilan fungsi anchor ada pesan " Message: Call to undefined function anchor() "